Navelwort. This is a pretty ugly name. I don’t want to over sell this plant but really the name sets the expectations rather low so there’s really only one way to go from here. Thankfully it is quite pretty.
This plant grows in the dead spaces – between little gaps in walls, between the pavement and nearby wall, on the upended roots of a fallen and dying tree, even on a decaying log. With less competition for space the small succulent disc shaped leaves can grow comfortably. If that was all that you had to go on you could confuse it for the marsh pennywort. Thankfully the flowers are suitably different – the navelwort produces spears of flowers, a bit like floxgloves. The marsh pennywort’s flowers are below the leaves, tiny and inconspicuous.
The belly button reference in the name isn’t random, the leaf really does look like a navel. Something that helped me when IDing this little plant in the field – keep an eye out for it – it is in full flower at the moment and looks like little waterfalls down the wall.